1. Field of the Invention
The present invention relates to a driver circuit, which in an operating mode drives a component that supplies output power when a driving input signal exceeds a first threshold value, having a differential amplifier whose output signal controls the driving input signal, having a reference signal generator that supplies a reference input of the differential amplifier, and having an external feedback that applies a signal, which is dependent on the output signal, to a feedback input of the differential amplifier.
The invention further relates to a method for compensation of offset currents in such a driver circuit.
2. Description of the Background Art
A typical example of such a component is a laser diode in which a laser effect occurs only above a laser threshold. For a laser diode, the external feedback takes place by the radiated optical power of the laser diode and a photodiode that is connected to the feedback input and receives a portion of the radiated optical power. When the laser diode radiates a comparatively high optical power, the photodiode supplies a high photocurrent to the feedback input of the differential amplifier. This reduces the difference at the input of the differential amplifier, which reduces the output signal of the differential amplifier, and thereby reduces the optical power of the laser diode. Similarly, a relatively low radiated optical power leads to an increase in the difference and thereby to an increase in the optical power. The feedback thus closes a control loop, by which a stable optical power is established at a stable input signal difference in the steady state.
In this context, a signal difference corresponding to the quotient of the output signal and the gain of the differential amplifier is established between the reference input and the feedback input.
Ideal differential amplifiers deliver reproducibly identical output signals for specific reference signal values and thus possess a reproducibly stable characteristic curve. In real differential amplifiers, however, shifts in the characteristic curves arise through offset currents of the differential amplifiers. The offset currents can be represented in an equivalent schematic as an additive offset of the reference signal.
In the case of a driver circuit with a differential amplifier that has such an offset, therefore, signal distortion occurs at the reference input. In the absence of countermeasures, such a signal distortion is stabilized by the external feedback. When the reference signal is switched off, the offset current alone acts as a reference signal in the equivalent schematic. Under certain circumstances, namely when the laser threshold is exceeded, the external feedback then establishes a final output power even though the switched off reference signal generator should likewise reduce the output power to zero.
For this reason, such behavior is always problematic when small output power levels are to be established, as is the case for a laser diode in a CD or DVD unit in read operation, for example.